Cooling method for fuel cell

ABSTRACT

A cooling method for a fuel cell using a heat exchanger is provided. The method comprises the steps of: providing an ion exchanger in a circulation system for the cooling fluid; decreasing ion concentration in the cooling fluid by circulating the cooling fluid through the fuel cell and the ion exchanger when the temperature of the cooling fluid is below a thermostat operating temperature; distributing a portion of the cooling fluid discharged from the fuel cell, mixing the portion of the cooling fluid returning from the heat exchanger with another portion of the cooling fluid whose ion concentration has been decreased in the previous step, and returning the cooling fluid mixture to the fuel cell, when the temperature of the cooling fluid is approaching the thermostat operating temperature; and cooling the fuel cell by circulating the cooling fluid through the fuel cell and the heat exchanger after the temperature of the cooling fluid reaches the thermostat operating temperature. By using this cooling method, the cooling fluid having a high conductivity is not distributed to the fuel cell.

RELATED APPLICATIONS

This application is a Divisional application of U.S. patent applicationSer. No. 10/272,345 filed Oct. 15, 2002, now U.S. Pat. No. 7,070,873,which claims priority to Japanese Patent Application No. 2001-318158filed 16 Oct. 2001 and Japanese Patent Application No. 2001-318159 filed16 Oct. 2001 in Japan. The contents of the aforementioned applicationsare hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a cooling method for a fuel cell in which heatproduced during power generation in the fuel cell is dissipated bycirculating a cooling fluid and by using a heat exchanger.

2. Background Art

Some fuel cells mounted in fuel cell vehicles are formed by stacking aplurality of fuel cell units, each of which comprises: a solid polymerelectrolyte membrane, such as a solid polymer ion-exchange membraneelectrode or the like, sandwiched between an anode and a cathode; and apair of conductive separators holding the solid polymer electrolytemembrane therebetween, wherein a fuel gas, e.g., hydrogen gas, issupplied to the anode of each of the separators and an oxidizing gas,e.g., oxygen-containing air, is supplied to the cathode of each of theseparators so as to generate electric power. In this type of fuel cell,hydrogen ions produced at the anode by a catalytic reaction move to thecathode through the solid polymer electrolyte membrane, and react withoxygen at the cathode to generate electric power.

In this type of fuel cell, because heat is produced during powergeneration and because the temperature of the fuel cell should bemaintained within an appropriate range, the fuel cell is cooled byflowing a cooling fluid through cooling fluid passages formed in theseparators of each of the fuel cell units and by dissipating heat of thecooling fluid using a heat exchanger. In a case in which such a coolingsystem is employed, it is necessary to control the amount of heatdissipated in order to avoid excessive cooling of the fuel cell when thefuel cell is operated in a cold environment, or is being operated so asto generate a small amount of power. As one conventional heatdissipation control method, a method in which cooling fluid circuits areswitched depending on temperature using a thermostat valve is known. Inthis method, the cooling fluid circuits are switched so as to circulatethe cooling fluid in the fuel cell while detouring the cooling fluidaround the heat exchanger when the temperature of the cooling fluid isin a low temperature zone, i.e., below a temperature at which thethermostat valve operates (hereinafter, this temperature is referred toas the thermostat operating temperature), and so as to circulate thecooling fluid in the fuel cell while also circulating it through theheat exchanger when the temperature of the cooling fluid is in a hightemperature zone, i.e., above the thermostat operating temperature.

When such a cooling system in which the separators are directly cooledby the cooling fluid is employed, because the conductivity of thecooling fluid must be maintained to be low in order to preventelectrical leakage via the cooling fluid, the cooling fluid is made toflow through an ion exchanger or the like to remove ions contained inthe cooling fluid so that the conductivity of the cooling fluid ismaintained to be low.

However, in the system in which heat dissipation amount is controlled byswitching the cooling fluid circuits using the thermostat valve, becausethe cooling fluid sits in the heat exchanger and in passages forcirculating the cooling fluid through the heat exchanged when thecooling fluid circulates while detouring around the heat exchanger atlow temperature, the conductivity of the sitting cooling fluid may beincreased due to ions dissolved from the heat exchanger or the passages.If the conductivity of the cooling fluid sitting in the heat exchangeror the passages is increased under low temperature conditions, thecooling fluid which has been sitting in the heat exchanger and which hasa high conductivity may flow into the fuel cell when the cooling fluidcircuits are switched upon completion of warm up.

Conventionally, in order to avoid such a problem, the heat exchanger andcirculation pipes for the cooling fluid were made of material from whichonly a small amount of ions may dissolve; however, the materialrestricts the shape or manufacturing method of the heat exchanger,whereby the heat exchanger could be large, heavy, and expensive.Alternatively, the inside of the heat exchanger or the like may becoated in order to suppress ion dissolution; however, ions may dissolvewhen the coating is degraded.

SUMMARY OF THE INVENTION

Based on the above circumstances, an object of the present invention isto provide a cooling method for a fuel cell in which increase in theconductivity of the cooling fluid is suppressed by applying anappropriate treatment to a portion of the cooling fluid when thethermostat operating temperature is approached.

A further object of the present invention is to provide a cooling methodfor a fuel cell in which the cooling fluid does not sit in the heatexchanger, and also to provide a cooling method in which increase in theconductivity of the cooling fluid is prevented even when the temperatureof the cooling fluid is in a low range.

In order to achieve the above object, a first aspect of the presentinvention provides a cooling method for a fuel cell, in which heatproduced during power generation in the fuel cell is dissipated bycirculating a cooling fluid and by using a heat exchanger and athermostat provided for switching flow passages of the cooling fluiddepending on the temperature thereof, the method comprising the stepsof: providing an ion exchanger, for removing ions contained in thecooling fluid, in a circulation system for the cooling fluid; decreasingion concentration in the cooling fluid by circulating the cooling fluidthrough the fuel cell and the ion exchanger when the temperature of thecooling fluid is below a thermostat operating temperature at which thethermostat valve of the thermostat is operated; distributing a portionof the cooling fluid discharged from the fuel cell, mixing the portionof the cooling fluid returning from the heat exchanger with anotherportion of the cooling fluid whose ion concentration has been decreasedin the previous step, and returning the cooling fluid mixture to thefuel cell, when the temperature of the cooling fluid is approaching thethermostat operating temperature; and cooling the fuel cell bycirculating the cooling fluid through the fuel cell and the heatexchanger after the temperature of the cooling fluid reaches thethermostat operating temperature.

According to the above cooling method, even if ion concentration in thecooling fluid in the heat exchanger is increased when the temperature ofthe cooling fluid is below the thermostat operating temperature, it ispossible to distribute the cooling fluid to the fuel cell afterdecreasing its ion concentration by mixing with another portion of thecooling fluid whose ion concentration has been decreased.

A second aspect of the present invention provides a cooling method for afuel cell, in which heat produced during power generation in the fuelcell is dissipated by circulating a cooling fluid and by using a heatexchanger and a thermostat provided for switching flow passages of thecooling fluid depending on the temperature thereof, the methodcomprising the steps of: providing an ion exchanger, for removing ionscontained in the cooling fluid, in a circulation system for the coolingfluid; removing ions contained in the cooling fluid by circulating thecooling fluid through the fuel cell and the ion exchanger when thetemperature of the cooling fluid is below a thermostat operatingtemperature at which the thermostat valve of the thermostat is operated;removing ions contained in the cooling fluid in the heat exchanger bycirculating a portion of the cooling fluid through the heat exchangerand the ion exchanger, when the temperature of the cooling fluid isapproaching the thermostat operating temperature; and cooling the fuelcell by circulating the cooling fluid through the fuel cell and the heatexchanger after the temperature of the cooling fluid reaches thethermostat operating temperature.

According to the above cooling method, even if ion concentration in thecooling fluid in the heat exchanger is increased when the temperature ofthe cooling fluid is below the thermostat operating temperature, it ispossible to decrease ion concentration in the cooling fluid containing alarge number of ions by removing ions by the time the temperature of thecooling fluid reaches the thermostat operating temperature.

The method of the second aspect may further comprise the steps of:providing a conductivity sensor for measuring the conductivity of thecooling fluid; and stopping the step of removing ions contained in thecooling fluid in the heat exchanger by circulating a portion of thecooling fluid through the heat exchanger and the ion exchanger when theconductivity of the cooling fluid is decreased below a predeterminedvalue.

According to the above cooling method, the cooling fluid is notdistributed from the heat exchanger to the ion exchanger after theconductivity of the cooling fluid is decreased below a predeterminedvalue; therefore, it is possible to increase the amount of the coolingfluid which is discharged from the heat exchanger and is distributed tothe fuel cell.

A third aspect of the present invention provides a cooling method for afuel cell, in which heat produced during power generation in the fuelcell is dissipated by circulating a cooling fluid and by using a heatexchanger and a first and second thermostats provided for switching flowpassages of the cooling fluid depending on the temperature thereof, themethod comprising the steps of: providing an ion exchanger, for removingions contained in the cooling fluid, in a circulation system for thecooling fluid; removing ions contained in the cooling fluid bycirculating the cooling fluid through the fuel cell and the ionexchanger when the temperature of the cooling fluid is below a firstthermostat operating temperature at which the thermostat valve of thefirst thermostat is operated; removing ions contained in the coolingfluid in the heat exchanger by circulating a portion of the coolingfluid through the heat exchanger and the ion exchanger, when thetemperature of the cooling fluid is above the first thermostat operatingtemperature and below a second thermostat operating temperature at whichthe thermostat valve of the second thermostat is operated; and coolingthe fuel cell by circulating the cooling fluid through the fuel cell andthe heat exchanger after the temperature of the cooling fluid reachesthe second thermostat operating temperature.

According to the above cooling method, even if ion concentration in thecooling fluid in the heat exchanger is increased when the temperature ofthe cooling fluid is below the first thermostat operating temperature,it is possible to decrease ion concentration in the cooling fluidcontaining a large number of ions by removing ions by the time thetemperature of the cooling fluid reaches the second thermostat operatingtemperature.

A fourth aspect of the present invention provides a cooling method for afuel cell, in which heat produced during power generation in the fuelcell is dissipated by circulating a cooling fluid and by using a firstheat exchanger, the method comprising the steps of: providing an ionexchanger, for removing ions contained in the cooling fluid, in acirculation system for the cooling fluid; removing ions contained in thecooling fluid in the first heat exchanger by circulating a portion ofthe cooling fluid through the first heat exchanger and the ion exchangerwhen the temperature of the cooling fluid is below a predeterminedvalue; and cooling the fuel cell by circulating the cooling fluidthrough the fuel cell and the first heat exchanger when the temperatureof the cooling fluid is above the predetermined value.

According to the above cooling method, because ions contained in thecooling fluid in the first heat exchanger are removed by circulating aportion of the cooling fluid through the first heat exchanger and theion exchanger when the temperature of the cooling fluid is below thepredetermined value, the cooling fluid does not sit in the first heatexchanger, and it is possible to decrease ion concentration in thecooling fluid in the first heat exchanger.

The method of the fourth aspect may further comprise the steps of:providing a counterflow type second heat exchanger between the firstheat exchanger and the ion exchanger; and transferring heat from thecooling fluid pre-entering the first heat exchanger to the cooling fluiddischarged from the first heat exchanger when the temperature of thecooling fluid is below the predetermined value.

According to the above cooling method, it is possible to suppress heatdissipation amount of the first heat exchanger even when a portion ofthe cooling fluid is circulated through the first heat exchanger and theion exchanger when the temperature of the cooling fluid is below thepredetermined value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram showing a first embodiment of a fuel cellsystem to which the cooling method for a fuel cell according to thepresent invention is applicable, and specifically showing, as a firstpart, a flow state of the cooling fluid in a low temperature zone.

FIG. 2 is a system diagram showing the first embodiment of a fuel cellsystem, and specifically showing, as a second part, a flow state of thecooling fluid in the low temperature zone.

FIG. 3 is a system diagram showing the first embodiment of a fuel cellsystem, and specifically showing a flow state of the cooling fluid in ahigh temperature zone.

FIGS. 4A to 4C are diagrams showing the operation of a thermostat valveused in the first embodiment.

FIG. 5 a diagram showing the flow characteristics of the thermostatvalve used in the first embodiment.

FIG. 6 is a system diagram showing a second embodiment of a fuel cellsystem to which the cooling method for a fuel cell according to thepresent invention is applicable, and specifically showing, as a firstpart, a flow state of the cooling fluid in a low temperature zone.

FIG. 7 is a system diagram showing the second embodiment of a fuel cellsystem, and specifically showing, as a second part, a flow state of thecooling fluid in the low temperature zone.

FIG. 8 is a system diagram showing the second embodiment of a fuel cellsystem, and specifically showing a flow state of the cooling fluid in ahigh temperature zone.

FIG. 9 a diagram showing the flow characteristics of the thermostatvalve used in the second embodiment.

FIG. 10 is a system diagram showing a third embodiment of a fuel cellsystem to which the cooling method for a fuel cell according to thepresent invention is applicable, and specifically showing, as a firstpart, a flow state of the cooling fluid in a low temperature zone.

FIG. 11 is a system diagram showing the third embodiment of a fuel cellsystem, and specifically showing, as a second part, a flow state of thecooling fluid in the low temperature zone.

FIG. 12 is a system diagram showing the third embodiment of a fuel cellsystem, and specifically showing a flow state of the cooling fluid in ahigh temperature zone.

FIG. 13 is a system diagram showing a fourth embodiment of a fuel cellsystem to which the cooling method for a fuel cell according to thepresent invention is applicable, and specifically showing a flow stateof the cooling fluid in a low temperature zone.

FIG. 14 is a system diagram showing the fourth embodiment of a fuel cellsystem, and specifically showing a flow state of the cooling fluid in ahigh temperature zone.

FIGS. 15A and 15B are diagrams showing the operation of a thermostatvalve used in the fourth embodiment.

FIG. 16 is a system diagram showing a fifth embodiment of a fuel cellsystem to which the cooling method for a fuel cell according to thepresent invention is applicable, and specifically showing a flow stateof the cooling fluid in a low temperature zone.

FIG. 17 is a system diagram showing the fifth embodiment of a fuel cellsystem, and specifically showing a flow state of the cooling fluid in ahigh temperature zone.

FIG. 18 is a diagram showing an example of the structure of a secondheat exchanger 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first to third embodiments of the cooling method for a fuel cell,according to the present invention, will be explained with reference toFIGS. 1 to 12. The cooling method for a fuel cell in each of thefollowing embodiments will be explained as being applied to a fuel cellto be mounted in a fuel cell vehicle.

First Embodiment

The first embodiment of the cooling method for a fuel cell, according tothe present invention, will be explained with reference to FIGS. 1 to 5.

FIGS. 1 to 3 are schematic system diagrams showing a cooling system fora fuel cell mounted in a fuel cell vehicle.

A fuel cell 1 consists of a stack formed by stacking a plurality of fuelcell units, each of which comprises, for example: a solid polymerelectrolyte membrane, such as a solid polymer ion-exchange membraneelectrode or the like, sandwiched between an anode and a cathode; and apair of separators holding the solid polymer electrolyte membranetherebetween. In the fuel cell 1, when hydrogen gas is supplied to theanode and oxygen-containing air is supplied to the cathode, hydrogenions produced at the anode by a catalytic reaction move to the cathodethrough the solid polymer electrolyte membrane, and electrically andchemically react with oxygen at the cathode to generate electric power.In FIG. 1, the supply system and the discharge system for hydrogen gasand air are not shown.

In the fuel cell 1, cooling fluid passages are formed in the separators.The temperature of the fuel cell 1 is controlled so as to be maintainedwithin a predetermined temperature range, e.g., from 70 to 80° C. afterthe fuel cell is warmed-up, by flowing a cooling fluid through thecooling fluid passages so as to directly cool the separators.

Next, a cooling fluid circulation system in which the cooling fluidflows will be explained. First, the cooling fluid circuit in a state, inwhich the cooling fluid for the fuel cell 1 requires no cooling becausethe temperature of the cooling fluid is low, i.e., the temperature ofthe cooling fluid is in a low temperature zone, will be explained. Asshown in FIG. 1, the cooling fluid discharged from the cooling fluidpassage outlet 1 a of the fuel cell 1 is drawn by a cooling fluid pump 2via a cooling fluid piping 21, flows into a thermostat valve 3 via acooling fluid piping 22 after being pressurized by the cooling fluidpump 2, flows into the cooling fluid passage inlet 1 b of the fuel cell1 via a cooling fluid piping 23, flows through the cooling fluidpassages in the fuel cell 1, and is again discharged from the coolingfluid passage outlet 1 a to circulate.

Next, the cooling fluid circuit, in a state, in which the cooling fluidfor the fuel cell 1 requires cooling because the temperature of thecooling fluid is high, i.e., the temperature of the cooling fluid is ina high temperature zone, will be explained.

As shown in FIG. 3, the cooling fluid discharged from the cooling fluidpassage outlet 1 a of the fuel cell 1 and pressurized by the coolingfluid pump 2 flows into a radiator 4 via a cooling fluid piping 24branching from the cooling fluid piping 22. The radiator 4 is anair-cooled type heat exchanger in which the cooling fluid is cooled byheat dissipation using natural ventilation or forced draft by a fan. Thecooling fluid cooled through the radiator 4 flows into the thermostatvalve 3 via a cooling fluid piping 25, and flows into the cooling fluidpassage inlet 1 b of the fuel cell 1 via the cooling fluid piping 23 tocirculate.

As shown in FIGS. 1 and 3, either when the temperature of the coolingfluid is in the low temperature zone, or when it is in the hightemperature zone, a portion of the cooling fluid flowing through thecooling fluid piping 23 flows into an ion exchanger 5 via a coolingfluid piping 26 and an orifice 27. The ion exchanger 5 is filled withion exchange resin in order to remove ions contained in the coolingfluid so as to decrease the conductivity of the cooling fluid. Theorifice 27 is a regulating orifice which regulates the flow rate of thecooling fluid flowing into the ion exchanger 5 at a predetermined rate.The cooling fluid, from which ions have been removed by the ionexchanger 5, flows into the cooling fluid piping 23 via a cooling fluidpiping 28, and is drawn by the cooling fluid pump 2 to circulate.

As explained above, in the first embodiment of the cooling fluidcirculation system, either when the temperature of the cooling fluid isin the low temperature zone, or when in the high temperature zone,because a portion of the cooling fluid circulating through the fuel cell1 always flows through the ion exchanger 5 which removes ions, it ispossible to maintain the ion concentration in the cooling fluidcirculating through the fuel cell 1 to be below a predetermined level;thus, the conductivity of the cooling fluid can be maintained to bebelow a predetermined level, and the electrical insulation performanceof the cooling fluid in the fuel cell 1 can be ensured.

In the first embodiment of the cooling system, the cooling fluid circuitduring the low temperature zone and the cooling fluid circuit during thehigh temperature zone are switched to each other using the thermostatvalve 3. Now, the operation of the thermostat valve 3 for switching thecooling fluid circuit will be explained with reference to the schematicdrawings, i.e., FIGS. 4A to 4C.

The thermostat valve 3 comprises: a housing 31 defining a valve chambertherein; a partition 33 dividing the valve chamber into two valvechambers, i.e., a first valve chamber 34 and a second valve chamber 35;and a valve body 37 operable to open or close a communication hole 33 aformed in the partition 33, so as to connect or disconnect the firstvalve chamber 34 and the second valve chamber 35. In addition, thethermostat valve 3 has a thermostat (not shown) therein, which operatesthe valve body 37 in accordance with the temperature of the coolingfluid flowing through the valve chamber. The thermostat valve 3 operatesas follows: the valve body 37 closes the communication hole 33 a, asshown in FIG. 4A (hereinafter, this state is referred to as thecompletely closed state), when the temperature of the cooling fluid isbelow a temperature at which the thermostat operates (hereinafter simplyreferred to as the thermostat operating temperature), i.e., when thetemperature of the cooling fluid is in the low temperature zone; and thevalve body 37 is separated from the communication hole 33 a, as shown inFIG. 4C (hereinafter, this state is referred to as the completely openstate), when the temperature of the cooling fluid exceeds the thermostatoperating temperature, i.e., when the temperature of the cooling fluidis in the high temperature zone. The thermostat will not be explainedherein because it is well known.

Cooling fluid pipings 22 and 23 are connected to the first valve chamber34, and a cooling fluid piping 25 is connected to the second valvechamber 35.

Accordingly, when the temperature of the cooling fluid is in the lowtemperature zone and the thermostat valve 3 is in the completely closedstate as shown in FIG. 4A, the cooling fluid flows into the first valvechamber 34 through the cooling fluid piping 22, and flows out to thecooling fluid piping 23. In this state, the cooling fluid does not flowthrough the cooling fluid piping 25 because the cooling fluid piping 25is closed at the second valve chamber 35.

On the other hand, when the temperature of the cooling fluid is in thehigh temperature zone and the thermostat valve 3 is in the completelyopen state as shown in FIG. 4C, the cooling fluid flows into the secondvalve chamber 35 through the cooling fluid piping 25, and flows out tothe cooling fluid piping 23. In this state, the cooling fluid does notflow into the first valve chamber 34 through the cooling fluid piping 22because the valve body 37 closes the inlet 22 a from the cooling fluidpiping 22 into the first valve chamber 34.

As explained above, the thermostat valve 3 switches the cooling fluidcircuit in such a way that the valve body 37 operates so as to open orclose the communication hole 33 a and the inlet 22 a from the coolingfluid piping 22.

When the temperature of the cooling fluid is in the low temperaturezone, the cooling fluid circulates through the fuel cell 1 whiledetouring around the radiator 4, as mentioned above, and a portion ofthe cooling fluid sits in the radiator 4 and in the cooling fluidpipings 24 and 25. Therefore, in this state, ions may dissolve from theradiator 4 and the cooling fluid pipings 24 and 25, which leads to anincrease in the ion concentration of the sitting cooling fluid, andleads to an increase in the conductivity thereof. Accordingly, theelectrical insulation performance of the cooling fluid in the fuel cell1 could be degraded because a large amount of cooling fluid, which hasbeen sitting in the radiator 4 and in the cooling fluid pipings 24 and25, and which has a high conductivity, flows into the cooling fluidpassages in the fuel cell 1 through the cooling fluid piping 23 when thetemperature of the cooling fluid transitions from the low temperaturezone into the high temperature zone, and consequently the thermostatvalve 3 instantaneously switches from the closed state to the openstate.

As a countermeasure for the above problem, in the first embodiment, apredetermined temperature zone immediately below the thermostatoperating temperature is set to be as a pre-operation temperature zone,and when the temperature of the cooling fluid is in the pre-operationtemperature zone, a portion of the cooling fluid discharged from thefuel cell 1 is made to flow into the radiator 4, and the cooling fluiddischarged from the radiator 4 is made to return to the fuel cell 1after mixing with another portion of the cooling fluid which circulatesdetouring around the radiator 4 and which has a low ion concentration.As a result, an increase in the conductivity of the cooling fluidcirculating through the fuel cell 1 can be prevented, or at least can bereduced to be within an acceptable level for the fuel cell 1. After thecooling fluid in the radiator 4 and in the cooling fluid pipings 24 and25 is replaced by another portion of the cooling fluid having a lowconductivity, the cooling fluid circuit is completely switched to astate in the high temperature zone as described above.

As a specific measure to realize the above operation, the thermostatvalve 3 having the flow characteristics shown in FIG. 5 is used in thefirst embodiment. In FIG. 5 showing the flow characteristics, thehorizontal axis indicates the temperature of the cooling fluid, and thevertical axis indicates a flow rate ratio (Q2/Q1) defined by the ratiobetween the flow rate Q1 of the cooling fluid flows into the first valvechamber 34 from the cooling fluid piping 22 and the flow rate Q2 of thecooling fluid flows into the second valve chamber 35 from the coolingfluid piping 25. In the completely closed state (Q2=0) shown in FIG. 4A,the flow rate ratio is shown as 0%, and in the completely open state(Q1=0) shown in FIG. 4C, the flow rate ratio is shown as 100%.

In the flow characteristics of thermostat valve 3, the thermostatoperating temperature is set to T1, the low temperature zone is set tobe below the thermostat operating temperature T1, the high temperaturezone is set to be above the thermostat operating temperature T1, and thetemperature zone from temperature T0, which is below the thermostatoperating temperature T1 by a predetermined temperature, to thethermostat operating temperature T1 is set to be as the pre-operationtemperature zone. The thermostat valve 3 starts to open when thetemperature of the cooling fluid exceeds the temperature T0, the flowrate ratio gradually increases (for example, the flow rate ratio at T1is about 10%) in the pre-operation temperature zone, the flow rate ratiorapidly increases above temperature T1, and the completely open state isreached at temperature T2 (T0<T1<T2).

The thermostat valve 3 having the flow characteristics explained aboveoperates in the pre-operation temperature zone in such a way that thevalve body 37 is slightly away from the communication hole 33 a so as toslightly open the communication hole 33 a, and at the same time, thevalve body 37 is sufficiently away from the inlet 22 a from the coolingfluid piping 22 so as to widely open the inlet 22 a, as shown in FIG.4B. As a result, a portion of the cooling fluid flowing into the firstvalve chamber 34 from the cooling fluid piping 22 at a large flow rateand another portion of the cooling fluid flowing into the first valvechamber 34 from the cooling fluid piping 25 via the second valve chamber35 at a small flow rate are mixed, and then the mixed cooling fluidflows out to the cooling fluid piping 23.

Accordingly, as the temperature of the cooling fluid rises from belowtemperature T0 toward the thermostat operating temperature T1, a portionof the cooling fluid, which has been circulating through the fuel cellwhile detouring around the radiator 4 when the temperature of thecooling fluid is below temperature T0, and which has a low ionconcentration (for simplicity, hereinafter, this portion of the coolingfluid is referred to as the circulating cooling fluid at lowtemperature), is mixed with another portion of the cooling fluid, whichhas been sitting in the radiator 4 and in the cooling fluid pipings 24and 25 when the temperature of the cooling fluid is below temperatureT0, and which has a relatively high ion concentration (for simplicity,hereinafter, this portion of the cooling fluid is referred to as thesitting cooling fluid), as shown in FIG. 2; however, because the flowrate of the sitting cooling fluid remains low until the temperature ofthe cooling fluid reaches the thermostat operating temperature T1, theion concentration of the mixed cooling fluid is only slightly higherthan that of the circulating cooling fluid at low temperature;therefore, an increase in the conductivity of the cooling fluidcirculating through the fuel cell 1 can be prevented, or at least it canbe reduced to be within an acceptable level for the fuel cell 1.

In addition, during an operation in the pre-operation temperature,because a portion of the cooling fluid flowing through the cooling fluidpiping 23 flows into the ion exchanger 5 via the cooling fluid piping26, a portion of the mixed cooling fluid is subjected to ion removal bythe ion exchanger before flowing into the fuel cell 1; therefore, theconductivity of the cooling fluid circulating through the fuel cell 1can return to a low value within a short period.

If the flow characteristics of the thermostat valve 3 and the entiresystem are set so that the cooling fluid in the radiator 4 and in thecooling fluid pipings 24 and 25 is replaced by the circulating coolingfluid at low temperature when the temperature of the cooling fluidreaches the thermostat operating temperature T1, the cooling fluidhaving a low ion concentration, i.e., having a low conductivity, can bedistributed to the fuel cell 1 immediately after the transition into thehigh temperature zone when the temperature of the cooling fluidtransitions from the low temperature zone into the high temperaturezone.

As explained above, according to the first embodiment of the coolingmethod for a fuel cell, the electrical insulation performance of thecooling fluid in the fuel cell 1 can always be maintained within anacceptable range, regardless of the temperature of the cooling fluid.

Second Embodiment

Next, the second embodiment of the cooling method for a fuel cell,according to the present invention, will be explained with reference toFIGS. 6 to 9.

FIGS. 6 to 9 are schematic system diagrams showing a cooling system fora fuel cell in the second embodiment.

The features in the second embodiment of the cooling system for a fuelcell which are different from that in the first embodiment will beexplained below.

In the second embodiment, the thermostat valve 3 has the flowcharacteristics shown in FIG. 9. In the flow characteristics ofthermostat valve 3, the thermostat operating temperature is set to T1′,the low temperature zone is set to be below the thermostat operatingtemperature T1′, the high temperature zone is set to be above thethermostat operating temperature T1′, and the pre-operation temperaturezone is not provided. Therefore, the thermostat valve 3 immediatelyswitches from the completely closed state to the completely open statewhen the temperature of the cooling fluid exceeds the thermostatoperating temperature T1′.

On the other hand, a slightly upstream point, as viewed from thethermostat valve 3, on the cooling fluid piping 25 and a point, betweenthe orifice 27 and the ion exchanger 5, on the cooling fluid piping 26,is connected by a cooling fluid piping 41 provided with an open/closevalve 6.

In addition, a temperature sensor 7, measuring the temperature of thecooling fluid flowing into the cooling fluid passage inlet 1 b of thefuel cell 1, is provided at a slightly upstream point, as viewed fromthe thermostat valve 3, on the cooling fluid piping 23. The open/closevalve 6 is controlled in accordance with the temperature of the coolingfluid measured by the temperature sensor 7. More specifically, theopen/close valve 6 is controlled to be closed when the temperature ofthe cooling fluid measured by the temperature sensor 7 is belowtemperature T3, and the open/close valve 6 is controlled to open whenthe temperature of the cooling fluid measured by the temperature sensor7 exceeds temperature T3. Temperature T3 is set to be below thethermostat operating temperature T1′ by a predetermined temperature(T3<T1′<T2).

Other features are the same as in the first embodiment; therefore, thesame reference symbols are appended to the same elements, andexplanations thereof are omitted.

Next, the cooling method for a fuel cell according to the secondembodiment will be explained.

As in the first embodiment, when the temperature of the cooling fluid isin the low temperature zone, the cooling fluid circulates through thefuel cell 1 while detouring around the radiator 4, as mentioned above,and a portion of the cooling fluid sits in the radiator 4 and in thecooling fluid pipings 24 and 25, in the second embodiment. Therefore, inthis state, ions may dissolve from the radiator 4 and the cooling fluidpipings 24 and 25, which leads to an increase in the ion concentrationof the sitting cooling fluid, and leads to an increase in theconductivity thereof. Accordingly, the electrical insulation performanceof the cooling fluid in the fuel cell 1 may be degraded because a largeamount of cooling fluid, which has been sitting in the radiator 4 and inthe cooling fluid pipings 24 and 25, and which has a high conductivity,flows into the cooling fluid passages in the fuel cell 1 through thecooling fluid piping 23 when the temperature of the cooling fluidtransitions from the low temperature zone into the high temperaturezone, and consequently the thermostat valve 3 instantaneously switchesfrom the closed state to the open state.

As a countermeasure for the above problem, in the second embodiment,when the temperature of the cooling fluid approaches the thermostatoperating temperature T1′, i.e., before the temperature of the coolingfluid reaches the thermostat operating temperature T1′, a portion of thecooling fluid is made to circulate through the radiator 4 and the ionexchanger 5 so as to remove ions contained in the cooling fluid whichhas been sitting in the radiator 4 and in the cooling fluid pipings 24and 25, and which has a high conductivity, by the ion exchanger 5, andto decrease the ion concentration thereof; and then the cooling fluidcircuit is switched to the state in the high temperature zone.

As a specific measure to realize the above operation, one thermostatvalve 3 and the open/close valve 6 are used in the second embodiment.

The flow of the cooling fluid will be explained below in accordance withthe change in the temperature of the cooling fluid with reference toFIGS. 6 to 9.

FIG. 6 shows the flow of the cooling fluid when the temperature of thecooling fluid is in the low temperature zone and below temperature T3.In this state, both the thermostat valve 3 and the open/close valve 6are in the completely closed state. Accordingly, the cooling fluidcirculates in the cooling fluid circuit in the low temperature zone. Thecooling fluid circuit in the low temperature zone is the same as in thefirst embodiment, and the cooling fluid flows from the cooling fluidpassage outlet 1 a of the fuel cell 1, through the cooling fluid piping21, through the cooling fluid pump 2, through the cooling fluid piping22, through the thermostat valve 3, through the cooling fluid piping 23,through the cooling fluid passage inlet 1 b of the fuel cell 1, andthrough the fuel cell 1, while a portion of the cooling fluid flowsthrough the cooling fluid piping 26, through the orifice 27, through theion exchanger 5, and through the cooling fluid piping 28.

When the temperature of the cooling fluid is in the low temperaturezone, and it exceeds the temperature T3 and is below the thermostatoperating temperature T1′, the thermostat valve 3 is maintained in thecompletely closed state, and only the open/close valve 6 transitions tothe open state. Upon opening of the open/close valve 6, the coolingfluid circulates in the cooling fluid circuit in the low temperaturezone, while a portion of the cooling fluid flowing into the coolingfluid piping 22 starts to flow through the cooling fluid piping 24, andthrough the radiator 4, as shown in FIG. 7. As a result, the coolingfluid, which has been sitting in the radiator 4 and in the cooling fluidpipings 24 and 25 before the open/close valve 6 opens, and which has arelatively high ion concentration, flows to an upstream point, as viewedfrom the ion exchanger 5, on the cooling fluid piping 26 through theopen/close valve 6 and through the cooling fluid piping 41, and is mixedwith another portion of the cooling fluid with a low ion concentrationflowing through the cooling fluid piping 23, through the cooling fluidpiping 26, and into the ion exchanger 5 in which ions contained in themixed cooling fluid are removed. Accordingly, it is possible to decreasethe ion concentration in the cooling fluid, which has been sitting inthe radiator 4 and in the cooling fluid pipings 24 and 25 before theopen/close valve 6 opens, and which has a relatively high ionconcentration, while preventing the cooling fluid with a high ionconcentration from flowing into the fuel cell 1.

Advantageously, the operating temperature T3 of the open/close valve 6is set to be an appropriate value, the flow rate of the cooling fluidflowing through the cooling fluid piping 41 is set to be an appropriatevalue, and a flow restriction device, such as an orifice, is provided onthe cooling fluid piping 41, if necessary, so that the cooling fluid inthe radiator 4 and in the cooling fluid pipings 24 and 25 is completelyreplaced before the temperature of the cooling fluid reaches thethermostat operating temperature T1′.

When the temperature of the cooling fluid exceeds the thermostatoperating temperature T1′, the thermostat valve 3 starts to open, andwhen the temperature of the cooling fluid exceeds the temperature T3,the thermostat valve 3 transitions to the completely open state. Duringthis transition operation, the open/close valve 6 is maintained in theopen state. FIG. 8 shows the flow state of the cooling fluid in thisstate in which the cooling fluid circulates through the cooling fluidcircuit in the high temperature zone. The cooling fluid circuit in thehigh temperature zone is the same as in the first embodiment, and thecooling fluid flows from the cooling fluid passage outlet 1 a of thefuel cell 1, through the cooling fluid piping 21, through the coolingfluid pump 2, through the cooling fluid piping 22, through the coolingfluid piping 24, through the radiator 4, through the cooling fluidpiping 25, through the thermostat valve 3, through the cooling fluidpiping 23, through the cooling fluid passage inlet 1 b of the fuel cell1, and through the fuel cell 1, while a portion of the cooling fluidflows through the cooling fluid piping 26, through the orifice 27,through the ion exchanger 5, and through the cooling fluid piping 28.Because the open/close valve 6 opens, a portion of the cooling fluidflowing through the cooling fluid piping 25 flows to the ion exchanger 5via the cooling fluid piping 41 and the open/close valve 6.

When the cooling fluid starts to flow though the cooling fluid circuitin the high temperature zone, the cooling fluid in the radiator 4 andthe cooling fluid in the cooling fluid pipings 24 and 25 have a low ionconcentration, and the entire cooling fluid circuit contains the coolingfluid having a low ion concentration; therefore, the cooling fluidhaving a low ion concentration, i.e., having a low conductivity, can bedistributed to the fuel cell 1 immediately after the temperature of thecooling fluid transitions into the high temperature zone.

Thus, according to the second embodiment of the cooling method for afuel cell, the electrical insulation performance of the cooling fluid inthe fuel cell 1 can always be maintained within an acceptable range,regardless of the temperature of the cooling fluid.

In the second embodiment, the open/close valve 6 is maintained in theopen state, and a portion of the cooling fluid flowing through thecooling fluid pipings 24 and 25 is maintained to be distributed to theion exchanger 5 when the temperature of the cooling fluid exceeds thethermostat operating temperature T1′, i.e., transitions into the hightemperature zone; however, it is not necessary to distribute the coolingfluid to the ion exchanger 5 via the cooling fluid piping 41 after theion concentration, or the conductivity, of the cooling fluid dischargedfrom the radiator 4, decreases below a predetermined value.

Therefore, a conductivity sensor 8 measuring the conductivity of thecooling fluid flowing through the cooling fluid pipings 24 and 25 may beprovided at an upstream point, as viewed from the open/close valve 6, onthe cooling fluid piping 41 so that the open/close valve 6 is closed,and so that the ion removing treatment by circulating the cooling fluidthrough the radiator 4 and the ion exchanger 5, when the conductivity ofthe cooling fluid measured by the conductivity sensor 8 decreases belowa predetermined value. According to this measure, it is possible todecrease the amount of the cooling fluid which has been cooled by theradiator 4, and which flows detouring around the fuel cell 1. In otherwords, more cooling fluid which has been cooled by the radiator 4 can bedistributed to the fuel cell 1; therefore, the fuel cell 1 can be moreeffectively cooled.

Third Embodiment

Next, the third embodiment of the cooling method for a fuel cell,according to the present invention, will be explained with reference toFIGS. 10 to 12. The basic concept of the third embodiment of the coolingmethod for a fuel cell is the same as that in the second embodiment, andthese embodiments differ from each other in their specific measures.

In the third embodiment, as in the second embodiment, when thetemperature of the cooling fluid approaches the thermostat operatingtemperature T1′, i.e., before the temperature of the cooling fluidreaches the thermostat operating temperature T1′, a portion of thecooling fluid is made to circulate through the radiator 4 and the ionexchanger 5 so as to remove ions contained in the cooling fluid whichhas been sitting in the radiator 4 and in the cooling fluid pipings 24and 25, and which has a high conductivity, by the ion exchanger 5, andto decrease the ion concentration thereof; and then the cooling fluidcircuit is switched to the state in the high temperature zone; however,as a specific measure to realize the above operation, two thermostatvalves, i.e., a first thermostat valve 3A and a second thermostat valve3B, whose thermostat operating temperatures differ from each other, areused in the third embodiment.

The features in the third embodiment of the cooling system for a fuelcell which are different from those in the first embodiment will beexplained with reference to FIGS. 10 to 12. The same reference symbolsare appended to the same elements in the second embodiment, andexplanations therefor are omitted.

The second thermostat valve 3B is equivalent to the thermostat valve 3in the second embodiment. As in the second embodiment, the secondthermostat valve 3B is connected to the cooling fluid piping 22 disposeddownstream of the cooling fluid pump 2, to the cooling fluid piping 25disposed downstream of the radiator 4, and to the cooling fluid piping23 connected to the cooling fluid inlet 1 b of the fuel cell 1.

The cooling fluid piping 24 disposed upstream of the radiator 4 isconnected to the first valve chamber 34 of the first thermostat valve 3Avia the cooling fluid piping 42, and the cooling fluid piping 25disposed downstream of the radiator 4 is connected to the second valvechamber 35 of the first thermostat valve 3A via the cooling fluid piping43. The first valve chamber 34 of the first thermostat valve 3A isconnected to the ion exchanger 5 via the cooling fluid piping 44 havingthe orifice 27. In the first thermostat valve 3A, the cooling fluidpiping 44 out of the cooling fluid pipings 42 and 44 connected to thefirst valve chamber 34 is opened or closed by the valve body 37.

Although the thermostat operating temperature T1 b′ (the secondthermostat operating temperature) of the second thermostat valve 3B isthe same as that of the thermostat valve 3 in the second embodiment, thethermostat operating temperature T1 a′ (the first thermostat operatingtemperature) of the first thermostat valve 3A is set to be lower thanthe thermostat operating temperature T1 b′ of the second thermostatvalve 3B (T1 a′<T1 b′).

In the cooling system constructed as described above the secondthermostat valve 3B operates to switch the cooling fluid circuit fromone in the low temperature zone to the other in the high temperaturezone, and the first thermostat valve 3A functions as the open/closevalve 6 and the temperature sensor 7 in the second embodiment.

The flow of the cooling fluid will be explained below in accordance withthe change in the temperature of the cooling fluid.

FIG. 10 shows the flow of the cooling fluid when the temperature of thecooling fluid is in the low temperature zone and below the thermostatoperating temperature T1 a′ of the first thermostat valve 3A. In thisstate, both the thermostat valves 3A and 3B are in the completely closedstate. Accordingly, the cooling fluid circulates in the cooling fluidcircuit in the low temperature zone. The cooling fluid circuit in thelow temperature zone is the same as in the second embodiment, and thecooling fluid flows from the cooling fluid passage outlet 1 a of thefuel cell 1, through the cooling fluid piping 21, through the coolingfluid pump 2, through the cooling fluid piping 22, through the secondthermostat valve 3B, through the cooling fluid piping 23, through thecooling fluid passage inlet 1 b of the fuel cell 1, and through the fuelcell 1. At the same time, a portion of the cooling fluid flows throughthe cooling fluid piping 42, through the first valve chamber 34 of thesecond thermostat valve 3B, through the cooling fluid piping 44, throughthe orifice 27, through the ion exchanger 5 in which ions are removed,and through the cooling fluid piping 28, and is drawn by the coolingfluid pump 2 to circulate.

When the temperature of the cooling fluid is in the low temperaturezone, and it exceeds the thermostat operating temperature T1 a′ of thefirst thermostat valve 3A, and is below the thermostat operatingtemperature T1 b′ of the second thermostat valve 3B, the secondthermostat valve 3B is maintained in the completely closed state, andonly the first thermostat valve 3A transitions to the completely openstate. FIG. 11 shows the flow of the cooling fluid in this state inwhich the cooling fluid circulates in the cooling fluid circuit in thelow temperature zone as described above, and a portion of the coolingfluid stops to flow through the cooling fluid piping 42 and the firstthermostat valve 3A when the first thermostat valve 3A transitions intothe completely open state; instead, another portion of the cooling fluidflowing into the cooling fluid piping 22 starts to flow through thecooling fluid piping 24, and through the radiator 4. As a result, thecooling fluid, which has been sitting in the radiator 4 and in thecooling fluid pipings 24 and 25 before the first thermostat valve 3Aopens, and which has a relatively high ion concentration, flows throughthe cooling fluid pipings 25 and 43, through the second valve chamber 35and the first valve chamber 34 of the first thermostat valve 3A, throughthe cooling fluid piping 44, through the orifice 27, and into the ionexchanger 5 in which ions contained in the cooling fluid are removed.Accordingly, it is possible to decrease the ion concentration in thecooling fluid, which has been sitting in the radiator 4 and in thecooling fluid pipings 24 and 25 before the first thermostat valve 3Aopens, and which has a relatively high ion concentration, whilepreventing the cooling fluid with a high ion concentration from flowinginto the fuel cell 1.

Advantageously, the thermostat operating temperature T1 a′ of the firstthermostat valve 3A is set to an appropriate value, and the flow rate ofthe cooling fluid flowing through the cooling fluid piping 44 is set toan appropriate value, so that the cooling fluid in the radiator 4 and inthe cooling fluid pipings 24 and 25 is completely replaced before thetemperature of the cooling fluid reaches the thermostat operatingtemperature T1 b′ of the second thermostat valve 3B.

When the temperature of the cooling fluid exceeds the thermostatoperating temperature T1 b′ of the second thermostat valve 3B, both ofthe thermostat valves 3A and 3B transition to the completely open state.FIG. 12 shows the flow of the cooling fluid in this state in which thecooling fluid circulates through the cooling fluid circuit in the hightemperature zone. The cooling fluid circuit in the high temperature zoneis the same as in the second embodiment, and the cooling fluid flowsfrom the cooling fluid passage outlet 1 a of the fuel cell 1, throughthe cooling fluid piping 21, through the cooling fluid pump 2, throughthe cooling fluid piping 22, through the cooling fluid piping 24,through the radiator 4, through the cooling fluid piping 25, through thesecond thermostat valve 3B, through the cooling fluid piping 23, throughthe cooling fluid passage inlet 1 b of the fuel cell 1, and through thefuel cell 1 to circulate. At the same time, a portion of the coolingfluid flowing through the cooling fluid piping 25 flows through thecooling fluid piping 43, through the second valve chamber 35 and thefirst valve chamber 34 of the first thermostat valve 3A, through thecooling fluid piping 44, through the orifice 27, and into the ionexchanger 5; and is drawn by the cooling fluid pump 2 via the coolingfluid piping 28 to circulate.

When the cooling fluid starts to flow though the cooling fluid circuitin the high temperature zone, the radiator 4 and the cooling fluidpipings 24 and 25 have the cooling fluid having a low ion concentration,and the entire cooling fluid circuit has the cooling fluid having a lowion concentration; therefore, the cooling fluid having a low ionconcentration, i.e., having a low conductivity, can be distributed tothe fuel cell 1 immediately after the temperature of the cooling fluidtransitions into the high temperature zone.

Thus, according to the third embodiment of the cooling method for a fuelcell, the electrical insulation performance of the cooling fluid in thefuel cell 1 can always be maintained within an acceptable rangeregardless of the temperature of the cooling fluid.

Modifications of the Above Embodiments

The present invention is not limited to the first to third embodimentsexplained above.

In the first to third embodiments explained above, the ion exchanger isdisposed parallel to the fuel cell; however, the present invention willprovide similar advantageous effects even when the ion exchanger isdisposed in series with the fuel cell.

Next, the fourth and fifth embodiments of the cooling method for a fuelcell, according to the present invention, will be explained withreference to FIGS. 13 to 18. The cooling method for a fuel cell in eachof the following embodiments will be explained as being applied to afuel cell to be mounted in a fuel cell vehicle.

Fourth Embodiment

The fourth embodiment of the cooling method for a fuel cell, accordingto the present invention, will be explained with reference to FIGS. 13to 15.

FIGS. 13 and 14 are schematic system diagrams showing a cooling systemfor a fuel cell mounted in a fuel cell vehicle.

A fuel cell 101 consists of a stack formed by stacking a plurality offuel cell units, each of which comprises, for example: a solid polymerelectrolyte membrane, such as a solid polymer ion-exchange membraneelectrode or the like, sandwiched between an anode and a cathode; and apair of separators holding the solid polymer electrolyte membranetherebetween. In the fuel cell 101, when hydrogen gas is supplied to theanode and oxygen-containing air is supplied to the cathode, hydrogenions produced at the anode by a catalytic reaction move to the cathodethrough the solid polymer electrolyte membrane, and electrically andchemically react with oxygen at the cathode to generate electric power.In FIG. 13, the supply system and the discharge system for hydrogen gasand air are not shown.

In the fuel cell 101, cooling fluid passages are formed in theseparators. The temperature of the fuel cell 101 is controlled so as tofall within a predetermined temperature range, e.g., from 70 to 80° C.after the fuel cell is warmed-up, by flowing a cooling fluid through thecooling fluid passages so as to directly cool the separators.

Next, a cooling fluid circulation system in which the cooling fluidflows will be explained. In the cooling system for a fuel cell in thisembodiment, the main flow passages are switched by a thermostat valve103; specifically, most of the cooling fluid is made to flow through thefuel cell 101 while detouring around a radiator 104 (a first heatexchanger) when the temperature of the cooling fluid is below atemperature (hereinafter, referred to as the low temperature zone) atwhich the thermostat valve operates (hereinafter, simply referred to asthe thermostat operating temperature), most of the cooling fluid is madeto flow through the radiator 104 in which the cooling fluid is cooled;then, is made to flow through the fuel cell 101 when the temperature ofthe cooling fluid exceeds the thermostat operating temperature(hereinafter, referred to as the low temperature zone).

The operation of the thermostat valve 103 will be explained withreference to FIG. 15.

The thermostat valve 103 comprises: a housing 131 defining a valvechamber therein; a partition 133 dividing the valve chamber into twovalve chambers, i.e., a first valve chamber 134 and a second valvechamber 135; and a valve body 137 operable to open or close acommunication hole 133 a formed in the partition 133, so as to connector disconnect the first valve chamber 34 and the second valve chamber135. In addition, the thermostat valve 103 has a thermostat (not shown)therein, which operates the valve body 137 in accordance with thetemperature of the cooling fluid flowing through the valve chamber. Thethermostat valve 103 operates as follows: the valve body 137 closes thecommunication hole 33 a, as shown in FIG. 15A (hereinafter, this stateis referred to as the completely closed state), when the temperature ofthe cooling fluid is below a temperature at which the thermostatoperates (hereinafter simply referred to as the thermostat operatingtemperature), i.e., when the temperature of the cooling fluid is in thelow temperature zone; and the valve body 137 is separated from thecommunication hole 133 a, as shown in FIG. 15B (hereinafter, this stateis referred to as the completely open state), when the temperature ofthe cooling fluid exceeds the thermostat operating temperature, i.e.,when the temperature of the cooling fluid is in the high temperaturezone. The thermostat will not be explained herein because it is wellknown.

Cooling fluid pipings 112 and 113 are connected to the first valvechamber 134, and a cooling fluid piping 119 is connected to the secondvalve chamber 135.

Accordingly, when the temperature of the cooling fluid is in the lowtemperature zone and the thermostat valve 103 is in the completelyclosed state as shown in FIG. 15A, the cooling fluid flows into thefirst valve chamber 134 through the cooling fluid piping 112, and flowsout to the cooling fluid piping 113. In this state, the cooling fluidpiping 119 is closed at the second valve chamber 135.

On the other hand, when the temperature of the cooling fluid is in thehigh temperature zone and the thermostat valve 103 is in the completelyopen state as shown in FIG. 15B, the cooling fluid flows into the secondvalve chamber 135 through the cooling fluid piping 119, and flows out tothe cooling fluid piping 113 via the first valve chamber 134. In thisstate, the cooling fluid does not flow into the first valve chamber 134through the cooling fluid piping 112 because the valve body 37 closesthe inlet 112 a from the cooling fluid piping 112 into the first valvechamber 34.

As explained above, the thermostat valve 103 switches the cooling fluidcircuit in such a way that the valve body 137 operates so as to open orclose the communication hole 133 a and the inlet 112 a from the coolingfluid piping 112.

Next, the cooling fluid circuit in a state, in which the cooling fluidfor the fuel cell 101 requires no cooling because the temperature of thecooling fluid is low, i.e., the temperature of the cooling fluid is in alow temperature zone, will be explained. As shown in FIG. 13, thecooling fluid discharged from the cooling fluid passage outlet 1 a ofthe fuel cell 101 is drawn by a cooling fluid pump 102 via the coolingfluid piping 111, flows into the thermostat valve 103 via the coolingfluid piping 112 after being pressurized by the cooling fluid pump 102,flows into the cooling fluid passage inlet 1 b of the fuel cell 101 viathe cooling fluid piping 113, flows through the cooling fluid passagesin the fuel cell 101, and is again discharged from the cooling fluidpassage outlet 101 a to circulate. This is the main flow circuit for thecooling fluid in the low temperature, and most of the cooling fluidcirculates through the fuel cell 101 via the main flow circuit.

When the temperature of the cooling fluid is in the low temperaturezone, a portion of the cooling fluid flowing through the cooling fluidpiping 113 flows into an ion exchanger 105 via a cooling fluid piping114 and an orifice 115. The ion exchanger 105 is filled with ionexchange resin in order to remove ions contained in the cooling fluid soas to decrease the conductivity of the cooling fluid. The orifice 115 isa regulating orifice which regulates the flow rate of the cooling fluidflowing into the ion exchanger 105 at a predetermined rate. The coolingfluid, from which ions have been removed by the ion exchanger 105, flowsinto a cooling fluid piping 111 via a cooling fluid piping 116, and isdrawn by the cooling fluid pump 102 to circulate. Accordingly, when thetemperature of the cooling fluid is in the low temperature zone, becausea portion of the cooling fluid circulating through the fuel cell 101always flows through the ion exchanger 105 which removes ions, it ispossible to maintain the ion concentration in the cooling fluidcirculating through the fuel cell 101 to be below a predetermined level;thus, the conductivity of the cooling fluid can be maintained to bebelow a predetermined level, and the electrical insulation performanceof the cooling fluid in the fuel cell 101 can be ensured.

In addition, when the temperature of the cooling fluid is in the lowtemperature zone, a portion of the cooling fluid flowing through thecooling fluid piping 112 flows into a primary fluid passage 106 a ofanother heat exchanger 106 (a second heat exchanger) via a cooling fluidpiping 117; then, the cooling fluid discharged from the primary fluidpassage 106 a flows into the radiator 104 via a cooling fluid piping118. The radiator 104 is an air-cooled type heat exchanger in which thecooling fluid is cooled by heat dissipation using natural ventilation orforced draft by a fan. The cooling fluid discharged from the radiator104 flows into a secondary fluid passage 106 b of the heat exchanger 106via the cooling fluid pipings 119 and 120. The cooling fluid dischargedfrom the secondary fluid passage 106 b returns to the cooling fluidpiping 111 via a cooling fluid piping 121 and an orifice 122, and thenis drawn by the cooling fluid pump 102 to circulate. Note that the flowrate of the cooling fluid flowing through the radiator 104 is limited toa small value by the orifice 122.

In this state, the cooling fluid does not flow from the cooling fluidpiping 119 into the first valve chamber 134 via the second valve chamber135 because the thermostat valve 103 is in the completely closed state,i.e., the communication hole 33 a of the thermostat valve 103 is closedby the valve body 137.

The heat exchanger 106 is a counterflow type heat exchanger in which thecooling fluid in the primary fluid passage 106 a flows oppositely to thecooling fluid in the secondary fluid passage 106 b, and in which heat istransferred between the cooling fluid flowing through the primary fluidpassage 106 a and the cooling fluid flowing through the secondary fluidpassage 106 b.

The advantageous effects, which can be obtained by making a portion ofthe cooling fluid flow through the radiator 104 when the temperature ofthe cooling fluid is in the low temperature zone, will be explainedbelow. When the temperature of the cooling fluid is in the lowtemperature zone, and if the cooling fluid is not distributed to theradiator 104, the ion concentration and the conductivity of the coolingfluid may be increased due to ions which may dissolve from the radiator104, because the cooling fluid is sitting in the radiator 104.Accordingly, the electrical insulation performance of the cooling fluidin the fuel cell 101 could be degraded because a large amount of coolingfluid, which has been sitting in the radiator 104, and which has a highconductivity, flows into the cooling fluid passages in the fuel cell 101through the cooling fluid piping 113 when the temperature of the coolingfluid transitions from the low temperature zone into the hightemperature zone, and consequently the thermostat valve 103 switchesfrom the closed state to the open state.

If a portion of the cooling fluid is made to flow through the radiator104 even when the temperature of the cooling fluid is in the lowtemperature zone, as in this embodiment, sitting of the cooling fluid inthe radiator 104 can be prevented, and consequently an increase in theion concentration in the cooling fluid in the radiator 104 can also beprevented. The cooling fluid discharged from the radiator 104 returns toan upstream point, as viewed from the cooling fluid pump 102, on thecooling fluid piping 11, at which the cooling fluid is mixed withanother portion of the cooling fluid flowing through the main flowpassage. Because a portion of the cooling fluid flowing through the mainflow passage flows through the ion exchanger 105 as explained above, itcan be said that a portion of the cooling fluid discharged from theradiator 104 flows through the ion exchanger 105. That is, a portion ofthe cooling fluid flows through the radiator 104 and through the ionexchanger 105 when the temperature of the cooling fluid is in the lowtemperature zone, whereby ions contained in the cooling fluid in theradiator 104 are removed.

Accordingly, even when the temperature of the cooling fluid is in thelow temperature zone, it is possible to maintain the ion concentrationin the cooling fluid in the radiator 104 to be below an appropriatelevel; thus, the conductivity of the cooling fluid can be maintained tobe below a predetermined level.

The advantageous effects, which can be obtained by transferring heatbetween the cooling fluid flowing into the radiator 104, i.e., thecooling fluid flowing through the primary fluid passage 106 a, and thecooling fluid discharged from the radiator 104, i.e., the cooling fluidflowing through the secondary fluid passage 106 b, will be explainedbelow.

If the heat exchanger 106 is not provided, the cooling fluid which washeated by the heat of the fuel cell 101 flows into the radiator 104, andthe cooling fluid is cooled by heat dissipation while flowing throughthe radiator 104; however, when the fuel cell 101 is operated in a coldenvironment or is operated so as to generate a small amount of power,the cooling fluid is cooled to quite a low temperature, and the coolingfluid having quite a low temperature returns to the upstream point asviewed from the cooling fluid pump 102 at which the cooling fluid ismixed with another portion of the cooling fluid flowing through the mainflow passage. As a result, although the flow rate of the cooling fluidhaving quite a low temperature is low, this cooling fluid stronglyinfluences the temperature of the cooling fluid flowing through the mainflow passage, the temperature of the cooling fluid supplied to the fuelcell 101 is decreased, and the fuel cell 101 may be excessively cooled;consequently, the temperature of the fuel cell 101 may be outside of anappropriate range.

On the other hand, when the heat exchanger 106 is provided, because thecooling fluid which was heated by the heat of the fuel cell 101 flowsinto the primary fluid passage 106 a, and the cooling fluid dischargedfrom the radiator 104 flows through the secondary fluid passage 106 b,the heat of the cooling fluid flowing through the primary fluid passage106 a is transferred to the cooling fluid flowing through the secondaryfluid passage 106 b. As a result, the temperature of the cooling fluidflowing through the primary fluid passage 106 a is decreased, and thetemperature of the cooling fluid flowing through the secondary fluidpassage 106 b is increased, whereby difference in temperatures thereofis decreased. As a result, the amount of heat dissipated from theradiator 104 is also decreased.

Accordingly, when there is provided the heat exchanger 106, a portion ofthe cooling fluid, which is mixed with another portion of the coolingfluid flowing through the main flow passage after flowing through theradiator 104, exerts little influence on the temperature of the coolingfluid flowing through the main flow passage; therefore, the temperatureof the cooling fluid supplied to the fuel cell 101 is not decreased, andthe fuel cell 101 will not be excessively cooled; consequently, thetemperature of the fuel cell 101 can be maintained within an appropriaterange.

Next, the cooling fluid circuit in a state, in which the cooling fluidfor the fuel cell 101 requires cooling because the temperature of thecooling fluid is high, i.e., the temperature of the cooling fluid is ina high temperature zone, will be explained.

In this state, because the thermostat valve 103 is in the completelyopen state as shown in FIG. 14, the cooling fluid discharged from thecooling fluid passage outlet 101 a of the fuel cell 101 and pressurizedby the cooling fluid pump 102 flows into a radiator 104 via the coolingfluid piping 112, the cooling fluid piping 117, the primary fluidpassage 106 a of the heat exchanger 106, and the cooling fluid piping118. The cooling fluid cooled through the radiator 104 flows into thethermostat valve 103 via the cooling fluid piping 119, and flows intothe cooling fluid passage inlet 101 b of the fuel cell 101 via thecooling fluid piping 113 to circulate. This is the main flow passage forthe cooling fluid in the high temperature zone through which most of thecooling fluid flows to circulate through the fuel cell 101. When thetemperature of the cooling fluid is in a high temperature zone, thecooling fluid does not flow into the first valve chamber 134 through thecooling fluid piping 112 because the inlet 112 a from the cooling fluidpiping 112 is closed.

Even when the temperature of the cooling fluid is in the hightemperature zone, a portion of the cooling fluid flowing through thecooling fluid piping 113 flows into the ion exchanger 105 via thecooling fluid piping 114 and the orifice 115. The cooling fluid, fromwhich ions have been removed by the ion exchanger 105, flows into thecooling fluid piping 111 via the cooling fluid piping 116, and is drawnby the cooling fluid pump 102 to circulate. Accordingly, even when thetemperature of the cooling fluid is in the high temperature zone,because a portion of the cooling fluid circulating through the fuel cell101 always flows through the ion exchanger 105 which removes ions, it ispossible to maintain the ion concentration in the cooling fluidcirculating through the fuel cell 101 to be below a predetermined level;thus, the conductivity of the cooling fluid can be maintained to bebelow a predetermined level, and the electrical electrical insulationperformance of the cooling fluid in the fuel cell 101 can be ensured.

When the temperature of the cooling fluid is in the high temperaturezone, a portion of the cooling fluid discharged from the radiator 104flows into the secondary fluid passage 106 b of the heat exchanger 106via the cooling fluid piping 120, and returns to the cooling fluidpiping 111 via the cooling fluid piping 121 and the orifice 122. Becausethe flow rate of the cooling fluid returning to the cooling fluid piping111 is regulated by the orifice 122, the amount of the cooling fluidreturning to the cooling fluid piping 111 is much less than that of thecooling fluid flowing through the main flow passage, and because theheat loss in the heat exchanger 106 is very small, the coolingperformance of the radiator 6 is hardly influenced.

As explained above, even when the temperature of the cooling fluidtransitions from the low temperature zone into the high temperaturezone, the thermostat valve 103 instantaneously switches from the closedstate to the open state, and consequently the cooling fluid circuitinstantaneously switches from one in the low temperature zone to theother in the high temperature zone, the cooling fluid having a low ionconcentration, i.e., having a low conductivity, can be distributed tothe fuel cell 101 immediately after the temperature of the cooling fluidtransitions into the high temperature zone because the ion concentrationin the cooling fluid in the radiator 104 is maintained to be low sincethe temperature of the cooling fluid was in the low temperature zone.

Therefore, according to the fourth embodiment of the cooling method fora fuel cell, the electrical insulation performance of the cooling fluidin the fuel cell 101 can always be maintained within an acceptable rangeregardless of the temperature of the cooling fluid, and the fuel cell101 will not be excessively cooled; consequently, the temperature of thefuel cell 101 can be maintained within an appropriate range.

Fifth Embodiment

The fifth embodiment of the cooling method for a fuel cell, according tothe present invention, will be explained with reference to FIGS. 16 and17.

FIGS. 16 and 17 are schematic system diagrams showing a cooling systemfor a fuel cell according to the fifth embodiment.

The features in the fifth embodiment of the cooling system for a fuelcell which are different from that in the fourth embodiment will beexplained below.

In the fifth embodiment, the ion exchanger 105 is not disposed betweenthe cooling fluid piping 111 and the cooling fluid piping 113, but isinstead disposed between the cooling fluid piping 111 and the coolingfluid piping 119. More specifically, a cooling fluid piping 124 havingan orifice 123 is connected to the cooling fluid piping 119 disposedimmediately upstream from the thermostat valve 103, and the coolingfluid piping 124 is further connected to the ion exchanger 105.Furthermore, the ion exchanger 105 is connected to the inlet of thesecondary fluid passage 106 b of the heat exchanger 106 via a coolingfluid piping 125, and the outlet of the secondary fluid passage 106 b isconnected to the cooling fluid piping 111 via a cooling fluid piping121.

Other features are the same as in the fourth embodiment; therefore, thesame reference symbols are appended to the same elements, andexplanations thereof are omitted.

Next, the flow of the cooling fluid in the fifth embodiment will beexplained below.

First, the flow of the cooling fluid when the temperature of the coolingfluid is in the low temperature zone will be explained with reference toFIG. 16. In this state, the main flow passage for the cooling fluid isthe same as in the fourth embodiment. The cooling fluid flows from thecooling fluid passage outlet 101 a of the fuel cell 101, through thecooling fluid piping 11, through the cooling fluid pump 102, through thecooling fluid piping 112, through the thermostat valve 103, through thecooling fluid piping 113, through the cooling fluid passage inlet 101 bof the fuel cell 101, and through the fuel cell 101.

In this state, a portion of the cooling fluid flows into the primaryfluid passage 106 a of the heat exchanger 106, and the cooling fluiddischarged from the primary fluid passage 106 a is distributed to theradiator 104 via the cooling fluid piping 118. The cooling fluiddischarged from the radiator 104 is distributed to the ion exchanger 105via the cooling fluid piping 119, the orifice 123, and the cooling fluidpiping 124. The cooling fluid, from which ions have been removed by theion exchanger 105, flows into the secondary fluid passage 106 b of theheat exchanger 106 via the cooling fluid piping 125, and the coolingfluid discharged from the secondary fluid passage 106 b returns to thecooling fluid piping 111 via the cooling fluid piping 121, and is drawnby the cooling fluid pump 102 to circulate. In other words, in the fifthembodiment, the ion exchanger 105 and the secondary fluid passage 106 bof the heat exchanger 106 are connected in series, and a certain portionof the cooling fluid flows into the secondary fluid passage 106 b afterflowing through the ion exchanger 105.

In the fifth embodiment, as in the previous embodiment, because aportion of the cooling fluid flows through the radiator 104 when thetemperature of the cooling fluid is in the low temperature zone, sittingof the cooling fluid in the radiator 104 can be prevented, andconsequently an increase in the ion concentration in the cooling fluidin the radiator 104 can also be prevented. In addition, because thecooling fluid discharged from the radiator 104 is made to flow throughthe ion exchanger 105, ions contained in the cooling fluid in theradiator 104 can be removed. Therefore, even when the temperature of thecooling fluid is in the low temperature zone, the ion concentration inthe cooling fluid in the radiator 104 can be suppressed to be low, whilethe ion concentration in the cooling fluid circulating through the fuelcell 101 can be maintained below a predetermined level.

In the fifth embodiment, as in the previous embodiment, because heat istransferred between relatively warm cooling fluid flowing into theradiator 104 and relatively cold cooling fluid discharged from theradiator 104, and because the difference in the temperatures thereof canbe decreased, the amount of heat dissipated from the radiator 104 can bedecreased. Accordingly, a decrease in the temperature of the coolingfluid distributed to the fuel cell 101 can be preferably prevented, andan excessive cooling of the fuel cell 101 can also be prevented;consequently, the temperature of the fuel cell 101 can be maintainedwithin an appropriate range, even when a portion of the cooling fluiddischarged from the radiator 104 and the heat exchanger 106 is mixedwith another portion of the cooling fluid flowing through the main flowpassage.

Next, the cooling fluid circuit in a state in which the temperature ofthe cooling fluid is in a high temperature zone will be explained withreference to FIG. 17. In this state, the main flow passage for thecooling fluid is the same as in the fourth embodiment. The cooling fluiddischarged from the cooling fluid passage outlet 101 a of the fuel cell101 flows through the cooling fluid pump 102, through the cooling fluidpiping 112, through the cooling fluid piping 117, through the primaryflow passage 106 a of the heat exchanger 106, through the cooling fluidpiping 118, through the radiator 104, through the cooling fluid piping119, through the thermostat valve 103, and through the cooling fluidpiping 113, and flows into the cooling fluid passage inlet 101 b of thefuel cell 101 to circulate.

In this state, as in the previous state, a portion of the cooling fluidflowing through the cooling fluid piping 119 flows into the ionexchanger 105 via the orifice 123 and the cooling fluid piping 124. Thecooling fluid, from which ions have been removed by the ion exchanger105, flows into the secondary flow passage 106 b of the heat exchanger106 via the cooling fluid piping 125. The cooling fluid discharged fromthe secondary flow passage 106 b returns to the cooling fluid piping 111via the cooling fluid piping 121, and is drawn by the cooling fluid pump102 to circulate. Accordingly, even when the temperature of the coolingfluid is in the high temperature zone, because a portion of the coolingfluid circulating through the fuel cell 101 always flows through the ionexchanger 105 which removes ions, it is possible to maintain the ionconcentration in the cooling fluid circulating through the fuel cell 101to be below a predetermined level; thus, the conductivity of the coolingfluid can be maintained to be below a predetermined level.

Therefore, in the fifth embodiment, as in the previous embodiment, evenwhen the temperature of the cooling fluid transitions from the lowtemperature zone into the high temperature zone, the thermostat valve103 instantaneously switches from the closed state to the open state,and consequently the cooling fluid circuit instantaneously switches fromone in the low temperature zone to the other in the high temperaturezone, the cooling fluid having a low ion concentration, i.e., having alow conductivity, can be distributed to the fuel cell 101 immediatelyafter the temperature of the cooling fluid transitions into the hightemperature zone because the ion concentration in the cooling fluid inthe radiator 104 is maintained to be low since the temperature of thecooling fluid was in the low temperature zone.

Furthermore, in the fifth embodiment, because the ion exchanger 105 andthe secondary fluid passage 106 b of the heat exchanger 106 areconnected in series, the amount of the cooling fluid, which was cooledby the radiator 104 and which circulates detouring around the fuel cell101, can be decreased compared with the fourth embodiment when thetemperature of the cooling fluid is in the high temperature zone. Inother words, more cooling fluid which has been cooled by the radiator104 can be distributed to the fuel cell 101; therefore, the fuel cell101 can be more effectively cooled.

As explained above, according to the fifth embodiment of the coolingmethod for a fuel cell, the electrical insulation performance of thecooling fluid in the fuel cell 101 can always be maintained within anacceptable range regardless of the temperature of the cooling fluid, andthe fuel cell 101 will not be excessively cooled; consequently, thetemperature of the fuel cell 101 can be maintained within an appropriaterange.

Modifications of the Above Embodiments

The present invention is not limited to the fourth and fifth embodimentsexplained above.

For example, the structure of the heat exchanger 106 is not limited; anda heat exchanger having a dual tube structure, in which the secondaryflow passage 106 b is disposed inside the primary flow passage 106 a, asshown in FIG. 18, may be employed.

Furthermore, the switching means to switch the cooling fluid circuitbetween one in the low temperature zone and the other in the hightemperature zone is not limited to a thermostat valve; and the means maycomprise a temperature sensor, and a valve which is controlled to beopen or closed in accordance with the output of the temperature sensor.

INDUSTRIAL APPLICABILITY

As explained above, according to the first aspect of the presentinvention, even if the ion concentration in the cooling fluid in theheat exchanger is increased when the temperature of the cooling fluid isbelow the thermostat operating temperature, this portion of the coolingfluid having a high ion concentration can be distributed to the fuelcell after being diluted with another portion of the cooling fluid whoseion concentration has been decreased; therefore, the followingadvantageous effects are obtainable: introduction of the cooling fluidhaving a high ion concentration into the fuel cell is prevented, and thefuel cell is maintained in an electrically stable state.

According to the second aspect of the present invention, even if the ionconcentration in the cooling fluid in the heat exchanger is increasedwhen the temperature of the cooling fluid is below the thermostatoperating temperature, the ion concentration of this portion of thecooling fluid having a high ion concentration can be decreased byremoving ions before the cooling fluid reaches the thermostat operatingtemperature; therefore, the following advantageous effects areobtainable: the cooling fluid having a low ion concentration can bedistributed to the fuel cell immediately after the cooling fluid startsto circulate through the fuel cell and the heat exchanger, and the fuelcell is maintained in an electrically stable state.

In addition, according to the present invention, because the coolingfluid is not distributed to the ion exchanger from the heat exchanger,and it is possible to increase the amount of the cooling fluiddistributed to the fuel cell after being discharged from the heatexchanger, the cooling capacity for the fuel cell can be increased.

According to the third aspect of the present invention, even if the ionconcentration in the cooling fluid in the heat exchanger is increasedwhen the temperature of the cooling fluid is below the operatingtemperature of the first thermostat, the ion concentration of thisportion of the cooling fluid having a high ion concentration can bedecreased by removing ions before the cooling fluid reaches theoperating temperature of the second thermostat; therefore, the followingadvantageous effects are obtainable: the cooling fluid having a low ionconcentration can be distributed to the fuel cell immediately after thecooling fluid starts to circulate through the fuel cell and the heatexchanger, and the fuel cell is maintained in an electrically stablestate.

According to the fourth aspect of the present invention, because aportion of the cooling fluid is made to flow through the first heatexchanger and the ion exchanger to remove ions contained in the coolingfluid in the first heat exchanger when the temperature of the coolingfluid is below a predetermined temperature, the cooling fluid does notsit in the first heat exchanger, and the ion concentration in thecooling fluid in the first heat exchanger can be decreased; therefore,the following advantageous effects are obtainable: the electricalinsulation performance of the cooling fluid in the fuel cell can bepreferably maintained, and the fuel cell is maintained in anelectrically stable state.

Furthermore, according to the present invention, even if a portion ofthe cooling fluid is made to flow through the first heat exchanger andthe ion exchanger to remove ions contained in the cooling fluid in thefirst heat exchanger when the temperature of the cooling fluid is belowthe predetermined temperature, the amount of heat dissipated in thefirst heat exchanger is maintained to be low; therefore, the coolingfluid is prevented from being excessively cooled, and an excessivecooling of the fuel cell can be prevented; consequently, the temperatureof the fuel cell can be maintained within an appropriate range.

1. A cooling method for a fuel cell, in which heat produced during powergeneration in said fuel cell is dissipated by circulating a coolingfluid and by using a heat exchanger and a thermostat provided forswitching flow passages of said cooling fluid depending on thetemperature thereof, the method comprising the steps of: providing anion exchanger, for removing ions contained in said cooling fluid, in acirculation system for said cooling fluid; removing ions contained insaid cooling fluid by circulating said cooling fluid through said fuelcell and said ion exchanger while allowing a portion of said coolingfluid to substantially stand in said heat exchanger when the temperatureof said cooling fluid is below a thermostat operating temperature atwhich the thermostat valve of said thermostat is operated; removing ionscontained in said cooling fluid in said heat exchanger by circulating aportion of said cooling fluid through said heat exchanger and said ionexchanger, when the temperature of said cooling fluid is approachingsaid thermostat operating temperature; and cooling said fuel cell bycirculating said cooling fluid through said fuel cell and said heatexchanger after the temperature of said cooling fluid reaches saidthermostat operating temperature.
 2. A cooling method for a fuel cellaccording to claim 1, further comprising the steps of: providing aconductivity sensor for measuring the conductivity of said coolingfluid; and stopping the step of removing ions contained in said coolingfluid in said heat exchange by circulating a portion of said coolingfluid through said heat exchanger and said ion exchanger when theconductivity of said cooling fluid is decreased below a predeterminedvalue.
 3. A cooling method for a fuel cell, in which heat producedduring power generation in said fuel cell is dissipated by circulating acooling fluid and by using a heat exchanger and first and secondthermostats provided for switching flow passages of said cooling fluiddepending on the temperature thereof, the method comprising the stepsof: providing an ion exchanger, for removing ions contained in saidcooling fluid, in a circulation system for said cooling fluid; removingions contained in said cooling fluid by circulating said cooling fluidthrough said fuel cell and said ion exchanger while allowing a portionof said cooling fluid to substantially stand in said heat exchanger whenthe temperature of said cooling fluid is below a first thermostatoperating temperature at which the thermostat valve of said firstthermostat is operated; removing ions contained in said cooling fluid insaid heat exchanger by circulating a portion of said cooling fluidthrough said heat exchanger and said ion exchanger, when the temperatureof said cooling fluid is above said first thermostat operatingtemperature and below a second thermostat operating temperature at whichthe thermostat valve of said second thermostat is operated; and coolingsaid fuel cell by circulating said cooling fluid through said fuel celland said heat exchanger after the temperature of said cooling fluidreaches said second thermostat operating temperature.